The present invention relates to the field of imaging, and in particular to forming an image of the above surface environment located on or above the surface of the water from underneath the water utilizing an imaging polarimeter to infer the two-dimensional slope of the water surface in the field-of-view of the image pixels.
U.S. Pat. No. 5,528,493 entitled Virtual Periscope discloses the use of an underwater camera, processor, and display system that reconstructs a fragmented image seen from below the surface into a view of the hemisphere atop the ocean surface. Unlike a conventional periscope that collects light from a position above the ocean surface, this U.S. patent discloses collecting skylight penetrating through the ocean surface with one or more upward-looking cameras positioned below the surface. The deeper the sensor, the larger the field of view available on the ocean surface. As the sensor's depth increases, however, it also receives less light. Because the underwater imaging system disclosed in this patent relies on peaked waves to collect rays from near the horizon, it cannot see as far as a conventional periscope extended above the wave tops.
According to Snell's Law, light above the surface is refracted below the surface into a cone of light that can be imaged by an upward-looking underwater camera equipped with a wide angle lens. However, the raw imagery collected by such a camera is unintelligible due to the distorting influence of waves on the ocean surface. The camera field of view must be about 130° to encompass the entire cone of refracted light and reconstruct the entire hemisphere above the surface.
Essential to producing useful through-surface imagery is the ability to map the ocean surface and characterize the perturbation of the light reaching the underwater sensor. Physical concepts of wave dynamics, Snell's Law, Fresnel transmission, and image blurring are key to this ability. Therefore, sophisticated signal processing techniques are required to extract useful information from the distorted imagery, especially for the portion of the scene of greatest interest near the horizon.
Therefore, there is a need for an improved underwater imaging system.